DE102017205279B3 - Cryostat assembly with a neck tube with a supporting structure and an outer tube surrounding the supporting structure to reduce the cryogen consumption - Google Patents

Abstract

A cryostat assembly (1) having a vacuum container (2) and a cryogenic container (3) disposed therein, the vacuum container having a neck tube (4) leading to the cryogenic container (3) having a supporting structure (4a) and a supporting structure (4a) surrounding Outer tube (4b), wherein via the neck tube, a connection can be made from the cryogenic container to a region outside the vacuum container, so that cryogenic fluid from the cryogenic container in an area outside the vacuum container can flow or vice versa, characterized in that the mechanical On the other hand, the parts of the neck tube which serve to establish a diffusion barrier between the interior of the cryogen container and the interior of the vacuum container are arranged spatially separated from one another and made of differently optimized materials, in other words, the cryogenic container is attached to the vacuum container the supporting structure bears the weight of the cryogenic vessel and is made of a material for which the ratio σ / θ of maximum permissible mechanical stress σ, σ> 100MPa, to the integral θ of the thermal conductivity λ over the temperature range ΔT is between 300K and 4K , with θ <300W / m, applies: σ / θ> 1/3 (MPa · m) / W, and that the outer tube is made of a material through which cryogenic fluid can not diffuse or only in virtually non-measurable amount , and which can be fluid-tightly connected to other components of the cryostat assembly so that the resulting integral leakage rate from the cryogen container into the vacuum container is less than 10 mbar · l / s. Thus, the originating from the neck pipes heat input into the cryogenic container can be significantly reduced.

Description

The invention relates to a cryostat with a vacuum container and a cryogenic container, which is disposed within the vacuum container, wherein the vacuum container has at least one neck tube with a supporting structure and an outer tube surrounding the supporting structure, wherein the neck tube leads to the cryogenic container and wherein via the neck tube spatial connection of an inner volume of the cryogen container to a region outside the vacuum container can be made so that cryogenic fluid from the cryogen container can flow into a region outside the vacuum container or vice versa.

Such a cryostat is approximately from the US-A 5,220,800 known.

Background of the invention

The present invention generally relates to the field of cooling of technical systems which are to be maintained at very low (= cryogenic) temperatures during operation. Such systems may include, for example, superconducting magnet arrangements, such as those used in the field of magnetic resonance, for example in MRI tomographs or NMR spectrometers. Such superconducting magnet assemblies are usually cooled with liquid helium as a cryogenic fluid.

An important feature of a superconducting NMR magnet system is the helium consumption during operation. On the one hand, helium consumption has an impact on the running costs that have to be spent on operating the system; On the other hand, the refill interval is largely dependent on helium consumption. The lower the refill interval, the longer the system can be operated trouble-free. With a constant refill interval, a system with lower helium consumption can also be made more compact, since the helium tank can be made smaller. As a result, the system becomes cheaper to produce, and the site requirements (e.g., room height) become lower. One of the development goals for superconducting magnet systems is therefore to reduce the consumption of liquid helium, which in the case of bath-cooled systems is equivalent to a reduction of the heat load on the helium tank.

In typical bath cryostats a large part of the total heat load on the helium tank is caused by heat conduction in the so-called "neck pipes". These neck pipes connect the tank in which the liquid cryogen is stored with the outside world. The cryogenic liquid can be replenished through the neck tubes and it can flow off (even at high flow rates, such as during a magnetic quench or sudden loss of vacuum insulation). The neck pipes are also needed to gain access to components in the tank (e.g., to the electrical connections of a solenoid). In many cryostats, the neck tubes also carry the weight of the tank. One of the biggest contributors to the total heat load on the helium tank comes from these neck pipes, with heat conduction in the pipe being the dominant mechanism.

Usually neck pipes are made particularly thin-walled; Wall thicknesses in the range of a few tenths of a millimeter are not uncommon. The neck pipes are typically made from a material with low thermal conductivity. The wall of the neck tube must not be gas permeable, so that the vacuum insulation of the cryostat does not become contaminated and therefore unusable. In addition, the neck tube material from the point of view of the connection technique must be well suited (weldability, solderability). In many cases, stainless steel is used.

The cited above US-A 5,220,800 discloses a generic Kryostatanordnung for an NMR magnet system with superconducting magnetic coil. The Kryostatanordnung has a double-walled neck tube, through whose annular gap a flow of helium can be done (see about there 4 ).

• „The manner in which the chambers 1 and 2, radiation shields 21 and 22, and cooling tank 23 are suspended in the cryostat 4 on suspension tubes 30 is depicted only schematically in 1. The connecting elements used are thinwalled tubes or bundles of three centering rods 26 each, a few millimeters in diameter, which have extremely low thermal conductivity and high tensile strength.“

For the mechanical structure and the attachment of outer and inner tube makes the US-A 5,220,800 the following information:

• In the chambers 1 and 2, radiation shields 21 and 22, and cooling tank 23 are suspended in the cryostat 4 on suspension tubes 1 , The connecting elements are each a few millimeters in diameter, which have extremely low thermal conductivity and high tensile strength. "

The mechanical suspension of the neck tube and the provision of a fluidic diffusion barrier are thus necessarily physically shared here, in particular by means of one and the same material. A separate optimization of the purely mechanical function of a suspension on the one hand and the fluidic function of a diffusion barrier on the other hand, such as with regard to the choice of material according to type and strength are therefore according to the teaching of US-A 5,220,800 neither possible nor planned.

Object of the invention

The present invention is over the prior art discussed above Detail considered relatively sophisticated and complex task, in a cryostat arrangement of the type described above with the least expensive technical means to reduce the originating from the neck pipes heat input into the cryogenic container -in the rule a helium tank- significantly.

Brief description of the invention

This object is solved by the features of patent claim 1. Advantageous embodiments thereof are mentioned in the dependent claims. In particular, this achieves the present invention in a surprisingly simple as well as effective manner in that in a cryostat arrangement of the type defined serving for a mechanical suspension of the cryogenic container on the vacuum container parts of the neck tube on the one hand and for establishing a diffusion barrier between the interior of the cryogen container and the On the other hand, portions of the neck tube serving as the inner space of the vacuum container are spatially separated from each other and made of differently optimized materials, respectively, such that the supporting structure supports the weight of the cryogen container and is made of a material having a maximum allowable mechanical stress ratio σ / θ σ, with σ> 100 MPa, to the integral θ of the thermal conductivity A over the temperature range ΔT between 300 K and 4 K, with θ <300 W / m, the following applies: σ / θ> 1/3 (MPa · m) / W, and that the outer tube made of a material igt, through which cryogenic fluid can not diffuse, or only in virtually non-measurable amount, and which can be fluid-tightly connected to other components of the Kryostatanordnung, so that the resulting integral leakage rate from the cryogenic container into the vacuum vessel is less than 10 ^-6 mbar · l / s.

The basic idea of the present invention is to make the generally double-walled neck tube so that the functions of mechanical attachment of the tank on the one hand and the production of tightness (minimal permeation of the cryogen through the neck tube wall) on the other hand are separated. This procedure allows a separate optimization.

The achievable with the present invention reduction of the heat load, however, not only for superconducting magnet systems has high relevance. For this reason, the invention is also useful in other areas of cryogenics (e.g., storage of cryogens such as helium or hydrogen).

The following advantages are achieved in particular with the present invention:

The heat load on the cryogenic tank can be significantly reduced by the thermal properties of its more efficient design of the neck tube.

In actively cooled systems, this allows the use of a cooler with lower cooling capacity, which, for example, beneficial to the power consumption and system costs.

In the case of bath-cooled systems, this leads to a reduction in the evaporation rate of the cryogenic fluid. On the one hand, this results in a significant reduction in operating costs, on the other hand, the time interval in which the cryogenic fluid (typically helium) has to be replenished increases, which reduces the disturbances associated with long-term nuclear magnetic resonance measurements and, overall, the availability of the system increased for NMR measurements.

Evaporating cryogen is in thermal contact with parts of the neck tube. This makes it possible to use the enthalpy of the cold gas to "intercept" heat flowing through heat conduction in the neck tube from the warm to the cold end. Corresponding non-fluid-tight support structures are in the prior art always in vacuum, which they lack the feature of a thermodynamically advantageous exhaust gas cooling.

Preferred embodiments and further developments of the invention

Very particular preference is given to an embodiment of the cryostat assembly according to the invention in which the supporting structure is in the form of an inner tube, and via the inner tube a spatial connection of the inner volume of the cryogen container can be made to a region outside the vacuum vessel, so cryogenic fluid from the cryogenic container can flow into an area outside the vacuum vessel or vice versa. A relatively large access to the cryogenic tank can then be created via the inner tube, via which e.g. electrical connections can be made to a superconducting magnet coil. In addition, such a large cross-section is available to allow high-flow cryogen to escape from the helium tank without excessive pressure rise from the cryogenic tank (e.g., in the case of a quench of superconducting magnet coil or in case of vacuum breakage).

A particularly simple development of this embodiment provides that the outer tube is in direct, preferably thermally well-conductive, contact with the inner tube. The diffusion barrier can be directly on the supporting pipe be applied. Inner and outer tubes are therefore in good thermal contact with each other. Cryogen that flows out through the inner tube primarily cools the inner tube but, because the tubes are in good thermal contact, is also able to absorb heat from the outer tube. The greatest advantages of this embodiment are simplicity of construction and thermodynamic efficiency.

In an alternative development, the outer tube can be spaced from the supporting structure and a gap can be left between the inner tube and the outer tube. This gap can be used in a variety of ways, as described in several of the following claims. For example, it is possible to allow cryogen to flow through the gap between the supporting structure and the outer tube, which enables particularly efficient cooling of the two tube walls. In addition, it is possible to use the gap as a pumping line for the operation of a Joule-Thompson cooler.

If the load-bearing structure in this embodiment is realized as an inner tube which is closed during normal operation, the advantages already mentioned above (eg large cross section for electrical connections and quenching / vacuum breakage) can be realized, while at the same time the "exhaust gas cooling" of the neck tube can be selected by appropriate choice Gap geometry can be optimized (eg small gap dimensions for high heat transfer coefficients).

Particularly preferred are variants of this development, in which the outer tube and the supporting structure are interconnected by a plurality of axially arranged, radially extending thermal bridges. As a result, even when the supporting structure is spaced from the outer tube, it is possible to ensure that there is good thermal contact between the two tubes and fluid flowing in the inner tube or gap. Such thermal bridges can be, for example, copper beryllium springs, which are thermally attached to the supporting structure. If the load-bearing structure is then pushed into the outer tube during system assembly, the copper-beryllium springs press against the inside of the outer tube, allowing good heat transfer.

In further advantageous variants, the gap between an inner tube and the outer tube at the end of the neck tube closer to the cryogenic container has a flow restrictor. If the inlet of the gap is protected by a throttle, and the space temperature-side outlet of the gap is dimensioned sufficiently large, the quench pressure in the dimensioning of the outer tube must not be taken into account, since no high pressure can build up in the gap. This makes it possible to make the outer tube, which tends to be made of a material with greater thermal conductivity (for example, stainless steel), to be particularly thin in order to produce sufficient tightness, which in turn minimizes the axial heat conduction in the outer tube.

The inner tube will typically have a wall thickness between 0.5 mm and 3 mm. To minimize the heat input into the cryogenic vessel, the wall thickness should be as thin as possible - just as thin as the mechanical strength requirements allow. For magnetic coils of the order of magnitude as the Applicant currently manufactures, this results in the range of wall thicknesses given above for typical neck tube diameters and the other materials mentioned below.

In further preferred embodiments of the invention, the supporting structure is made of plastic, preferably of fiber-reinforced plastic, in particular of GFRP, particularly preferably of the fiber composite material G10. In the case of GRP, the minimum achievable wall thickness is limited by manufacturing limitations. After all, fiberglass can not be made arbitrarily thin. G10 is a widely used material in cryotechnology, characterized by a particularly low ratio of thermal conductivity to strength. For mounting structures with low thermal conductivity G10 is therefore ideally suited. In addition, G10 is relatively cheap, and can be put into a variety of forms. Adhesive bonding makes it easy to connect metallic fittings to G10 components, or even better, to laminate them directly to the G10 component. The use of fiber composites opens up the possibility of orienting the fibers within the matrix to optimally utilize the anisotropic properties of the fiber, e.g. the tensile strength of a support tube in the axial direction is maximized.

In further advantageous embodiments of the invention, the supporting structure made of plastic at its two ends in each case a metal extension, preferably made of stainless steel, on. Typically, the inner tubes are connected to metallic parts in the cryostat (eg, the vacuum container or the cryogenic container). The installation of the cryostat is particularly easy if the plastic tube is already equipped with metallic sleeves at its ends. Then, during the assembly of the cryostat no metal-plastic connection must be made. This is advantageous because reliable metal-plastic joints are technologically difficult to manufacture during the assembly process, albeit easily can be integrated directly into the production of the supporting structure made of plastic (eg by direct casting of the metal sleeve with plastic), or can be readily prepared in a separate operation before mounting the cryostat (eg by gluing). With metal sleeves at the pipe ends can be easily welded. Welding is a process that can be easily integrated into the assembly process of a cryostat.

Particular preference is given to developments of these embodiments, in which the metal extensions each have a length between 20 mm and 100 mm, preferably about 50 mm, and a voltage cross-sectional area between 50 mm ² and 500 mm ² . As mentioned above, the metal sleeves are typically connected to other metallic parts in the cryostat (eg, the vacuum vessel or the cryogen vessel) by welding. The necessary during welding heat input could damage the plastic pipe, if there is no sufficient distance between the plastic pipe and the weld. With the dimensions given above it is ensured that the plastic pipe is not heated inadmissibly during the welding of the metal sleeve.

A further advantageous embodiment of the cryostat arrangement according to the invention is characterized in that the outer tube is made of metal, preferably of stainless steel. Metals are characterized by the fact that they can serve as a particularly effective diffusion barrier for cryogens. In addition, it is very easy to make reliable and fluid-tight connections between metals (e.g., by welding, brazing or soldering). Stainless steel is one of the most widely used materials in cryostat construction. Its low thermal conductivity and good weldability makes it predestined for the application described here. In addition, very thin-walled stainless steel tubes with wall thicknesses of a few tenths of a millimeter are easily commercially available, and can be produced cheaply by rolling and longitudinal welding. For cryostats of NMR magnet systems, the favorable magnetic properties and the low electrical conductivity are also advantageous. Thus, the heat conduction within the diffusion barrier (in addition to the favorable choice of material) can be further reduced.

In a preferred class of embodiments of the invention, so-called baffles are installed inside the neck tube, in particular within an inner tube and / or between the supporting structure and the outer tube, which absorb heat radiation and prevent convection. For the operation of the magnet system, it is advantageous in some respects, if the inner tube has a large diameter. This facilitates, for example, introducing components, such as power supply lines, signal lines, valve rods, etc., into the cryogenic vessel. With increasing diameter of the inner tube, however, the heat load on the cryogenic container increases - on the one hand by the larger cross-sectional area, which is available for the transport of heat radiation and heat conduction in the gas column, and on the other by a favorable geometry for the formation of convection vortices and thermoacoustic oscillations (Takonis oscillations). If baffles are installed in the neck tube, the heat radiation is largely isolated (the baffles act as gas-cooled radiation shields), and the formation of large convection vortices and thermoacoustic oscillations within the inner tube is prevented. The baffles prevent the mass flow. The heat conduction in the gas column is reduced because heat transfer resistances occur at each baffle.

Particularly preferred variants of this class of embodiments are characterized in that the baffles are designed to be foldable. A significant advantage of a large inner tube diameter is that in the case of a large heat load on the cryogen (eg by breaking the isolation vacuum, in a superconducting magnet system, for example by a quench) can not build up a large pressure in the cryogenic container, as sufficiently large outflow to To be available. However, if the free cross-section of the inner tube is reduced with baffles, this advantage would be eliminated. It is therefore particularly advantageous to make the baffles hinged, so that, as soon as it comes in the interior of the cryogenic container to a large pressure increase and therefore a large mass flow from the cryogenic container in the area outside the vacuum container, are folded away from the outflowing gas, and the Release the cross-section of the inner tube.

Also advantageous embodiments of the invention Kryostatanordnung with a tubular bearing structure in which the upper end of the inner tube is normally closed fluid-tight manner, in particular by a pressure relief valve or rupture disc, so that in normal operation effluent cryogen must flow through the gap between the inner tube and outer tube. Cold gas produced by the evaporation of the liquid cryogen in the cryogen vessel can still provide significant cooling power in the temperature range between the boiling point of the cryogen and the temperature of the vacuum vessel. The cold gas sweeps as it flows along the tube walls, absorbing heat that flows through heat conduction within the tube walls from the vacuum vessel to the cryogen vessel ("countercurrent cooling"). If the cold gas is passed through the gap between the inner and outer tubes, it comes in thermal contact with both the inner tube and the outer tube, allowing for particularly efficient countercurrent cooling. By suitable choice of the gap geometry, a good balance between a good heat transfer of the fluid with the walls of the gap and the pressure loss in the flowing fluid can be achieved.

Further advantageous embodiments of the invention are characterized in that the cryostat contains a Joule-Thompson cooler, is relaxed in the cryogen by means of a pump located outside of the vacuum vessel, and that the gap between the supporting structure and the outer tube part of the connecting line between the Joule Thompson cooler and the pump is. This in turn makes it possible to use the advantage already described above that the cold gas flowing out of the cryostat is used efficiently for reducing the heat load due to heat conduction. A Joule-Thompson cooler is particularly easy to integrate in a cryostat, since no separate pumping line must be provided. Absolute tightness between the pumping line (annular gap) and the cryogenic tank (or the volume in the inner neck tube) is not necessary. A low leakage current can be accepted if it is low compared to the current pumped out by the refrigerator.

Furthermore, in embodiments of the cryostat arrangement according to the invention, at least one bellows section may be present in the outer tube, so that the outer tube does not receive any axial forces. If the inner and outer tubes are made of different materials, it is very likely that these two materials have different thermal expansion coefficients. If the cryostat is cooled, therefore, large mechanical stresses would arise in the neck tube arrangement. These stresses can be counteracted if a bellows is installed in the outer tube. This bellows ensures that the tube remains largely stress-free.

Very particular preference is given to variants of the invention in which the cryostat arrangement is part of an apparatus for nuclear magnetic resonance, in particular for Magnetic Resonance Imaging (= MRI) or for magnetic resonance spectroscopy (= NMR), which preferably comprises a superconducting magnet arrangement. Superconducting magnets for MRI or NMR are usually cooled with liquid helium. However, the availability of helium and its price is a key factor in keeping He losses low.

Further advantages of the invention will become apparent from the description and the drawings. Likewise, according to the invention, the above-mentioned features and those which are still further developed can each be used individually for themselves or for a plurality of combinations of any kind. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

list of figures

• 1 eine schematische Vertikalschnittansicht einer ersten Ausführungsform der erfindungsgemäßen Kryostatanordnung;
• 2 eine schematische Vertikalschnittansicht einer zweiten Ausführungsform der erfindungsgemäßen Kryostatanordnung mit Baffles im Halsrohr;
• 3 eine schematische Vertikalschnittansicht einer dritten Ausführungsform der erfindungsgemäßen Kryostatanordnung mit einem Joule-Thompson-Kühler und einer Thermalbarriere im Kryogenbehälter;
• 4 eine schematische Vertikalschnittansicht einer vierten Ausführungsform der erfindungsgemäßen Kryostatanordnung mit einem Balg-Abschnitt im Halsrohr;
• 5a ein Diagramm des Wärmeleitfähigkeitsintegrals von Edelstahl über der Temperatur nach dem Stand der Technik; und
• 5b ein Diagramm des Wärmeleitfähigkeitsintegrals von G10 über der Temperatur nach dem Stand der Technik.

The invention is illustrated in the drawing and will be explained in more detail with reference to embodiments. Show it:

• 1 a schematic vertical sectional view of a first embodiment of the invention Kryostatanordnung;
• 2 a schematic vertical sectional view of a second embodiment of the invention Kryostatanordnung with baffles in the neck tube;
• 3 a schematic vertical sectional view of a third embodiment of the invention Kryostatanordnung with a Joule-Thompson cooler and a thermal barrier in the cryogenic container;
• 4 a schematic vertical sectional view of a fourth embodiment of the invention Kryostatanordnung with a bellows section in the neck tube;
• 5a a graph of the thermal conductivity integral of stainless steel over the temperature of the prior art; and
• 5b a graph of the thermal conductivity integral of G10 over the temperature of the prior art.

The 1 to 4 The drawings each show in a schematic view preferred embodiments of the cryostat arrangement according to the invention for storing a fluid cryogen, in particular for cooling a superconducting magnet arrangement.

Such a cryostat arrangement 1 has a vacuum tank 2 and a cryogenic container 3 on the inside of the vacuum container 2 is arranged, wherein the vacuum container 2 at least one neck tube 4 with a load-bearing structure 4a and a supporting structure 4a surrounding outer tube 4b, wherein the neck tube 4 to the cryogenic container 3 leads and being over the neck tube 4 a spatial connection of an inner volume of the cryogenic container 3 to an area outside the vacuum vessel 2 can be prepared so that cryogenic fluid from the cryogenic container 3 in an area outside the vacuum vessel 2 can flow or vice versa.

und dass das Außenrohr 4b aus einem Material gefertigt ist, durch welches kryogenes Fluid nicht oder nur in praktisch nicht-messbarer Menge diffundieren kann, und welches mit anderen Komponenten der Kryostatanordnung 1 fluiddicht verbunden werden kann, so dass die resultierende integrale Leckrate aus dem Kryogenbehälter 3 in den Vakuumbehälter 2 kleiner ist als 10^-6 mbar·l/s. The cryostat arrangement according to the invention 1 is distinguished from known devices of the prior art in that the mechanical suspension of the cryogenic container 3 on the vacuum container 2 serving parts of the neck tube 4 on the one hand and for the establishment of a diffusion barrier between the interior of the cryogenic container 3 and the interior of the vacuum container 2 serving parts of the neck tube 4 on the other hand arranged spatially separated from each other and are made of different optimized materials that the supporting structure 4a the weight of the cryogenic container 3 and is made of a material in which for the ratio σ / θ of maximum permissible mechanical stress σ, with σ> 100 MPa, to the integral θ of the thermal conductivity A over the temperature range ΔT between 300 K and 4 K, with θ <300 W / m, the following applies: $$\mathrm{\sigma }\text{/}\mathrm{\theta >}\text{3.1}\left(\text{MPa}\cdot \text{m}\right)\text{/ W}\text{.}$$

and that the outer tube 4b is made of a material through which cryogenic fluid can not diffuse or only in virtually non-measurable amount, and which with other components of the Kryostatanordnung 1 fluid-tight, so that the resulting integral leakage rate from the cryogenic container 3 in the vacuum tank 2 smaller than 10 ^-6 mbar · l / s.

In the in the 1 to 4 The drawing of embodiments of the invention shown is the supporting structure 4a formed in the form of an inner tube, and via the inner tube, a spatial connection of the inner volume of the cryogenic container 3 are made to a region outside the vacuum vessel 2, so that cryogenic fluid from the cryogenic vessel 3 into an area outside the vacuum vessel 2 can flow or vice versa. In this case, the outer tube 4b in direct, preferably thermally well-conductive, contact with the inner tube - which is not shown in the drawing. Alternatively - as in the 1 to 4 shown - the outer tube 4b spaced from the inner tube, and between the inner tube and the outer tube 4b A gap 4c stay open. In this case, the outer tube 4b and the inner tube is preferably interconnected by a plurality of axially arranged, radially extending thermal bridges.

As in the 1 . 2 and 4 can be seen, is flowing off cryogen through the annular gap 4c between the inner tube 4a and the outer tube 4b passed through in order to best use the enthalpy of the cold gas for the capture of the heat from the outer container (ie the vacuum container 2) along the neck tube 4 in the cryogenic tank 3 flows.

The outer tube 4b establishes the tightness for isolation vacuum. The wall thickness is designed so that the maximum differential pressure between the annular gap 4c and the isolation vacuum, and that no appreciable diffusion of the cryogen into the isolation vacuum occurs. The material is chosen so that a tight connection to other parts of the cryostat (eg the cover plate of the cryogenic tank 3 ) can be produced reliably and cheaply. The outer tube 4b can be made of stainless steel, for example, which has excellent welding properties. The wall thickness of this tube can be made very thin because the tube does not cover the entire weight of the cryogenic tank 3 (and the components contained therein) must record.

The inner tube 4a carries the weight of the cryogenic tank 3 , However, it does not have to be hermetically sealed, which is why a material can be chosen that is characterized primarily by the high ratio of mechanical strength and thermal conductivity. Here, for example, fiber-reinforced plastics come into question. For example, the load-bearing structure 4a in particular made of fiberglass, particularly preferably made of the fiber composite material G10.

The GRP pipe is connected at both ends with a stainless steel sleeve. Connections between GRP and stainless steel sleeve are known in the art. The stainless steel sleeve must have a certain minimum length (typically 50 mm).

The pictures of the 5a and 5b show the thermal conductivity integrals for stainless steel ( 5a ) and for the fiber composite G10 ( 5b ; Source: in each case JW Kooi, Caltech). As you can see, the thermal conductivity integral of stainless steel is about 30 times greater than that of G10. However, stainless steel is also considerably stronger than G10, which is why the ratio of strength to thermal conductivity has to be used as a key figure. The 0.2% yield strength of stainless steel that would be used for design is typically 360 MPa (for 1.4301); the tensile strength of G10 is about 270 MPa.

Even under the conservative assumption that G10 has a safety factor of 3 with respect to tensile strength and that stainless steel can be loaded down to its yield point, heat conduction through a stainless steel pipe is [(270/3 MPa) / (1 W / cm)]. / [(360 MPa) / (30 W / cm)] = 7.5 times larger than a GRP pipe with the same load capacity.

The supporting structure is preferred 4a made of plastic at its two ends in each case a metal extension 5a ' . 5a " , preferably made of stainless steel. The metal processes 5a ' . 5a " each have a length between 20 mm and 100 mm, preferably about 50 mm, and a voltage cross-sectional area between 50 mm ² and 500 mm ² .

As in 1 shown, in simple embodiments of the invention, the upper end of the inner tube 4a be closed fluid-tight in normal operation, in particular by a pressure relief valve or rupture disc 9 , so that in normal operation effluent cryogenic fluid through the gap 4c between inner tube 4a and outer tube 4b has to flow. The gap 4c between the inner tube 4a and the outer tube 4b indicates the cryogenic container 3 closer end of the neck tube 4 a flow restrictor 7 on.

In 2 an alternative embodiment of the invention is shown in which, within the neck tube 4 , in particular within the inner tube and / or between the supporting structure 4a and the outer tube 4b so-called Baffles 6 are installed, the heat radiation intercept and prevent convection. The baffles 6 are preferably designed to be foldable, so that in the case of a rapid outflow of the cryogen (eg in case of quenching) the cross section of the inner tube 4a release, and thus the pressure rise in the cryogenic tank 3 limit.

The annular gap 4c may also serve as a pumping line for supercooled systems in other embodiments of the invention. If the inlet of the pumping line is protected by a restrictor, the quench pressure must be taken into account when dimensioning the outer tube 4b not considered. Absolute tightness between pumping line (annular gap 4c ) and the cryogenic tank 3 (or the volume in the inner tube 4a ) is not necessary. A small leakage current can be accepted if it is small compared to the current pumped out by the refrigerator.

3 illustrates such an embodiment in which the cryostat contains a Joule-Thompson cooler JT in the cryogenic fluid by means of an outside of the vacuum container 2 befindlichem -in the drawing, not shown- pump is relaxed, the gap 4c between the supporting structure 4a and the outer tube 4b Part of the connecting line between the Joule-Thompson cooler JT and the pump is. In 3 a cryogenic tank is shown by means of a thermal barrier 20 is divided into two areas. The thermal barrier 20 is thermally insulating, but allows the pressure equalization between the two areas (eg a flexible membrane of thermally insulating material). Above the thermal barrier, for example, cryogen is at saturation at atmospheric pressure. Below the barrier, the cryogen is in the supercooled state (ie, atmospheric pressure but at a temperature below equilibrium temperature). For this, heat must be removed from the Joule-Thompson cooler JT. Such an arrangement is ideally suited for the operation of superconducting magnet coils at temperatures below 4.2 K.

In order to avoid a mechanical overdetermination of the system, can in the outer tube 4b a bellows section 8th be provided. This can be the outer tube 4b take no axial forces, and can therefore not be excessively strained by axial forces. Such a modified embodiment of the invention is in 4 shown.

Also conceivable are embodiments of the invention in which waiving the exhaust gas cooling in the annular gap 4c or the availability as a pumping line a variant is implemented without an annular gap, or in which the supporting structure - other than shown in the figures - not as a tube, but as a single rod or more rods is performed.

The features of all embodiments of the invention described above can be largely combined with each other.

Claims (15)

Cryostat arrangement (1) with a vacuum container (2) and a cryogenic container (3), which is arranged inside the vacuum container (2), wherein the vacuum container (2) has at least one neck tube (4), wherein the neck tube (4) to the cryogenic container ( 3), and wherein via the neck tube (4) a spatial connection of an inner volume of the cryogenic container (3) to a region outside the vacuum container (2) can be produced so that cryogenic fluid from the cryogenic container (3) in a region outside the vacuum container (2) can flow or vice versa, wherein the mechanical suspension of the cryogenic container (3) on the vacuum container (2) contributing parts of the neck tube (4) on the one hand and for establishing a diffusion barrier between the interior of the cryogen container (3) and the interior of the Vacuum container (2) serving parts of the neck tube (4) on the other hand arranged spatially separated from each other and each differently optimized materiali The functions of the mechanical fastening of the tank on the one hand and the production of the tightness, ie a minimum permeation of the cryogenic On the other hand, the neck tube (4) has a supporting structure (4a), the supporting structure (4a) having at each of its ends a metal extension (5a ', 5a "), the supporting structure (4a) bears the weight of the cryogenic container (3) and is made of a material for which the ratio σ / θ of maximum permissible mechanical stress σ, σ> 100 MPa, to the integral θ of the thermal conductivity over the temperature range ΔT is between 300 K and 4 K, with θ <300 W / m, applies: $$\mathrm{\sigma }\text{/}\mathrm{\theta >}\text{3.1}\left(\text{MPa}\cdot \text{m}\right)\text{/ W}\text{.}$$

wherein the supporting structure (4a) comprises a surrounding outer tube (4b), wherein the outer tube (4b) is made of a material through which cryogenic fluid does not diffuse or diffuses only in substantially non-measurable quantity, and which is combined with other components of the cryostat arrangement (4b). 1) is connected fluid-tight. Cryostat arrangement after Claim 1 , characterized in that the supporting structure (4a) is in the form of an inner tube, and via the inner tube, a spatial connection of the inner volume of the cryogen container (3) to a region outside the vacuum container (2) can be made, so cryogenic fluid from the cryogenic container (3) in a region outside the vacuum container (2) can flow or vice versa. Cryostat arrangement after Claim 2 , characterized in that the outer tube (4b) is in direct, preferably thermally well-conductive, contact with the inner tube. Cryostat arrangement after Claim 2 , characterized in that the outer tube (4b) is spaced from the inner tube, and between the inner tube and the outer tube (4b), a gap (4c) remains open. Cryostat arrangement after Claim 4 , characterized in that the outer tube (4b) and the inner tube are interconnected by a plurality of axially arranged, radially extending thermal bridges. Cryostat arrangement according to one of the preceding claims, characterized in that the supporting structure (4a) is made of plastic, preferably of fiber-reinforced plastic, in particular of fiberglass, more preferably of the fiber composite material G10. Cryostat arrangement according to one of the preceding claims, characterized in that the supporting structure (4a) is made of plastic and has at its two ends in each case a metal extension (5a ', 5a ") made of stainless steel. Cryostat arrangement after Claim 7 , characterized in that the metal extensions (5a ', 5a ") each have a length between 20 mm and 100 mm, preferably about 50 mm, and a voltage cross-sectional area between 50 mm ² and 500 mm ² . Cryostat arrangement according to one of the preceding claims, characterized in that the outer tube (4b) is made of metal, preferably of stainless steel. Cryostat arrangement according to one of the preceding claims, characterized in that within the neck tube (4), in particular within an inner tube (4a) and / or between the supporting structure (4a) and the outer tube (4b) Baffles (6) are installed, the heat radiation intercept and prevent convection. Cryostat arrangement after Claim 10 , characterized in that the baffles (6) are designed to be foldable. Cryostat arrangement after Claim 2 and optionally one of Claims 3 to 11 , characterized in that the upper end of the inner tube (4a) is closed fluid-tight in normal operation, in particular by a pressure relief valve or rupture disc (9), so flow in normal operation effluent cryogenic fluid through the gap between the inner tube (4a) and outer tube (4b) got to. Cryostat arrangement according to one of the preceding claims, characterized in that the cryostat contains a Joule-Thompson cooler (JT), in which cryogenic fluid is expanded by means of a pump located outside the vacuum container (2), and in that the gap (4c) between the supporting structure (4a) and the outer tube (4b) is part of the connecting line between the Joule-Thompson cooler (JT) and the pump. Cryostat arrangement after Claim 4 and optionally one of Claims 5 to 13 , characterized in that the gap (4c) between the inner tube (4a) and the outer tube (4b) at the cryogen container (3) closer to the end of the neck tube (4) has a flow restrictor (7). Cryostat arrangement according to one of the preceding claims, characterized in that in the outer tube (4b) at least one bellows section (8) is present, so that the outer tube (4b) receives no axial forces.

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